Mesoporous silicas containing carboxylic acid: Preparation,thermal degradation,and catalytic performance

The calcination and thermal degradation behaviors of surfactants in mesoporous silicas SBA-15 and MCM-41 were investigated by FT-IR, ¹³C CP/MAS NMR, TG/DTA, and GPC. It was found that carboxylic acid-containing products were generated as active components in the mesopores of SBA-15 and MCM-41 from the triblock copolymer (PEO)₂₀(PPO)₇₀(PEO)₂₀ and cetyltrimethylammonium bromide (CTAB), respectively; the latter materials were used as templates. The carboxylic acid-containing mesoporous silica obtained showed a catalytic activity for hydrolysis of sucrose. The acidity was evaluated by means of NaOH titration. The acidity sensitively depended on both the calcination temperature and the atmosphere; the maximum appeared at 150 °C in air for SBA-15 where the highest activity was observed. However, the product in MCM-41 showed a lower catalytic activity than that in SBA-15. The SBA-15 product was easily leached from the mesopores of SBA-15 into the solution, but the degree of leaching for MCM-41 was considerably smaller than that for SBA-15.     

Microporosity and mesoporosity of PPTA-derived carbons. Effect of PPTA thermal pretreatment

Isothermal treatments of the polyaramid fiber, [poly(p-phenylene terephthalamide)] (PPTA) in an inert atmosphere below its decomposition temperature are known to induce an important increase in char yield and modify the chemical composition and some other properties of the resulting chars. The objective of this work was to study the effect of this isothermal stage on the porous texture of chars and activated carbon fibers (ACFs) produced from PPTA. To this end, chars and ACFs were prepared by PPTA pyrolysis to 850 °C followed by CO₂ activation at 800 °C to various burn-offs (BOs), introducing or not an intermediate isothermal pre-treatment under the conditions (500 °C, 200 min) known to lead to a maximum increase in char yield. The porosity characteristics of the resulting chars and ACFs were comparatively investigated by adsorption of CO₂ (0 °C), and N₂ (−196 °C). The isothermal stage led to a char with enhanced micropore volume and wider micropores. The ACFs prepared from this char exhibited larger amounts of wide micropores and mesopores than those prepared from PPTA pyrolyzed at a constant heating rate.     

Influence of sulfur on the structural, surface properties and photocatalytic activity of sulfated TiO2

TiO₂ materials were prepared by sol–gel method and then impregnated with sulfuric acid and calcined using different temperatures and atmosphere (air and nitrogen). Systematic variation of these two experimental parameters makes possible to modulate the amount of surface sulfur from the impregnation procedure. The best photocatalyst for liquid phenol degradation was obtained after calcination at 700 °C in air, while gas toluene degradation optimum performance is obtained by calcination at 700 °C in nitrogen from 500 °C. Structural analysis of these materials by XRD, micro-Raman spectroscopy and FE-SEM shows that once calcined at 700 °C the material was a well-crystallized, high surface area anatase structure in all cases. The surface characterization by FTIR and XPS confirms the presence of a higher amount of sulfur species and acidic OH groups in samples partially calcined in nitrogen, and a low XPS O/Ti-atomic ratio with the O 1s peak shifted to higher binding energies (1.8 vs. 2 ± 0.1 and 530.4 eV vs. 529.8 eV, respectively, against the reference materials) for samples calcined at 700 °C, temperature at which most of sulfate species have been evolved. The paper presents an attempt to correlate the contribution of the observed structural defects within the anatase sub-surface layers and surface acidity to the different photoactivity behaviour exhibited for phenol liquid phase and toluene gas phase photodegradation.     

Fe-based electrocatalysts made with microporous pristine carbon black supports for the reduction of oxygen in PEM fuel cells

Fe-based catalysts for the oxygen reduction reaction at the cathode of polymer electrolyte membrane (PEM) fuel cells have been prepared using several highly microporous (defined as pores having a size <2 nm) carbon supports. The aim is to produce better performing catalysts as it is known that catalytic sites are hosted in the micropores of the carbon supports. All catalysts were loaded with a nominal Fe content of 0.2 wt% and were obtained by heat-treatment at 950 °C in pure NH₃ atmosphere. It is demonstrated, however, that the use of highly microporous carbon supports does not lead to improved catalytic activity, as originally expected, since the surface of these micropores is devoid of the nitrogen functionalities necessary to build the catalytic sites. Also, it is shown that for these microporous carbon supports, it is only the new micropores, i.e. those created during NH₃ etching at high temperature, that are capable of hosting catalytic sites.     

Solvent-free infiltration method for mesoporous SnO2 using mesoporous silica templates

Mesoporous tin oxide (SnO₂) materials, exhibiting high surface areas, crystalline frameworks and various mesostructures, were successfully obtained by a facile solvent-free infiltration method from mesoporous silica templates. Various kinds of mesoporous silica materials, such as KIT-6 (bicontinuous 3-D cubic, Ia3d), SBA-15 (2-D hexagonal, p6mm), SBA-16 (3-D cubic with cage-like pores, Im3m) and spherical mesoporous silica (disordered), were utilized as the hard templates. Tin precursor (SnCl₂ · 2H₂O, m.p. 310–311 K) was infiltrated spontaneously within the mesopores of silica templates by melting the precursor at 353 K without using any solvent. The heat-treatment of SnCl₂-infiltrated composite materials at 973 K under static air conditions and subsequent removal of silica templates by using HF result in the successful preparation of mesoporous SnO₂ materials. The mesostructures as well as the morphologies of mesoporous SnO₂ materials thus obtained were very similar with those of the mesoporous silica templates. The mesoporous SnO₂ materials exhibit high surface areas of 84–121 m²/g as well as high pore volumes in the range of 0.22–0.35 cm³/g. The present solvent-free infiltration method is believed to be a simple and facile way for the preparation of mesoporous materials via nano-replication from mesoporous silica templates.     

Incorporation of tin in different types of pores in SBA-15: Synthesis, characterization and catalytic activity

Mesoporous Sn-SBA-15 has been synthesized by three different methods such as conventional hydrothermal route, using cocatalyst NH₄F and in the presence of organosilane precursor. All the materials are thoroughly characterized by powder X-ray diffraction (XRD), SEM, TEM, N₂ sorption and surface area measurements, diffuse-reflectance UV–visible and FTIR spectroscopy, TG–DTA and elemental analysis through ICP. Nitrogen adsorption data, XRD patterns, and TEM observations suggests that the textural properties are retained during the isomorphous substitution of silicon by tin. ICP chemical analysis indicates that tin can be substituted in the range of Si/Sn = 69–162. UV–visible spectra of samples synthesized by the cocatalytic approach exhibit unique absorption band at 213 nm characteristics of tin atom substituted in the smaller pores (2–3 nm) located inside the walls of mesopores. Further, an additional band at 224 nm can be assigned to Sn atoms located in the distorted tetrahedral position along the primary mesopores. In contrary, only one absorption band centered at 224 nm is observed for all the samples synthesized by conventional hydrothermal as well as in the presence of organosilane precursor. ¹⁹F NMR spectra confirmed (no signal) the absence of occluded F⁻ ions in the samples made with NH₄F. Observed high catalytic activity in Baeyer–Villiger oxidation and Meerwin–Pondorf–Verly reduction under the liquid-phase conditions suggest the incorporation of a portion of tin in the smaller pores for the Sn-SBA-15 materials synthesized through cocatalyst method.     

Characterization of smectite after acid activation with microwave irradiation

The effect of acid activation under microwave (MW) irradiation on the textural and structural properties of a smectite from Maghnia (Algeria) was investigated.Sample aliquots were treated with 2 M hydrochloric acid for various times under variable irradiation power. The influence of activation parameters (MW power and time duration) on the textural and structural properties of the activated samples (crystallinity, specific surface area, pore volume, pore diameter, porosity, pore size distribution and morphology) have been studied.The prepared materials were characterized by X-ray diffraction, Differential Thermal and Thermo Gravimetric Analysis (DTA and TGA), Scanning Electron Microscopy (SEM) and nitrogen adsorption/desorption to obtain information about their structure and surface texture.The treated samples consisted of microporous and mesoporous structures. The specific surface area, pore diameter and pore volume of an adsorbent prepared by the microwave-heating method were 165.80 m²/g, 50 nm and 0.1919 cm³/g, respectively. The pore volumes of mesopores and micropores were 163.90 · 10^− 3 and 28.89 · 10^− 3 cm³/g, respectively. Furthermore, small pores were created in the prepared materials with microwave irradiation.     

Synthesis and characterization of large-pore FDU-12 silica

Large-pore FDU-12 (LP-FDU-12) silicas with face-centered cubic structures of spherical mesopores of diameter 16–26 nm were synthesized using Pluronic F127 triblock copolymer template. The materials were characterized using small-angle X-ray scattering and nitrogen adsorption. The initial temperature range suitable for LP-FDU-12 synthesis was shown to be much wider than that reported earlier (7–15 °C vs. 14–15 °C). However, the degree of structural ordering was lower and the pore size distribution was broader in case of temperatures of 7–9 °C. The removal of the template by calcination resulted in a significant shrinkage (unit-cell parameter decrease by 10–16%) and afforded samples with low adsorption capacity, unless the synthesis involved a hydrothermal treatment in an HCl solution, which reduced the shrinkage and significantly increased the pore volume. The selection of time and temperature of the acid treatment allowed us to adjust the pore diameter of LP-FDU-12. However, the broadening of pore size distribution was observed for excessively long treatments at 130 °C. The treatment at 130 °C allowed us to tailor the pore entrance size in LP-FDU-12.     

Accelerated formation of barium titanate by solid-state reaction in water vapour atmosphere

Barium titanate (BaTiO₃) powders were synthesized from commercially available raw materials (BaCO₃ and rutile) without particular mechanochemical processing by solid-state reactions in water vapour atmosphere. The formation rate of BaTiO₃ was accelerated by water vapour and single phase of BaTiO₃ was obtained by calcination at 700 °C for 4 h in water vapour atmosphere, though high temperature (850 °C for 2.5 h) was required by calcinations in air to complete the reaction. The formation kinetics followed the Valensi–Carter equation, which suggested that the reaction proceeded by a diffusion controlled process. The apparent activation energy for the formation of BaTiO₃ in air and water vapour atmosphere was estimated to be 361 ± 20 kJ/mol and 142 ± 17 kJ/mol, respectively. Water vapour is considered to enhance thermal decomposition of BaCO₃ and formation of BaTiO₃ by attacking surface Ti–O–Ti bonds in TiO₂, increasing partial pressure of Ba(OH)₂, and producing vacancies in the BaTiO₃ structure.     

Dispersion of Nano Nickel Particles over SBA-15 Modified by Carbon Films on Pore Walls

Mesoporous SBA-15 was prepared by using P123 as a template. The precursor with the template was calcined in an inert atmosphere so that carbon films might be formed in pores of SBA-15 due to the decomposition of template. The SBA-15C thus formed contained 3% C and exhibited similar pore structures as the SBA-15. Both SBA-15 and SBA-15C were used to support 20% nickel (by weight) via impregnation. It was found that doping with carbon films enhanced the dispersion of supported nickel. However, calcination at high temperatures before the reduction had a negative effect on the dispersion of nickel. The un-calcined 20%Ni/SBA-15C after the reduction in H₂ at 673 K exhibited the highest dispersion of nickel (42%) and smallest average particle size of about 2.4 nm, in the catalysts studied in this work. It was also the most active catalyst for the hydrogenation of toluene to methyl cyclohexane. Conversion of toluene could be detected even at room temperature and atmospheric pressure for the catalyst in a fix-bed reactor, and 100% conversion of toluene was reached when temperature was raised to 358 K.     

TiO2 photoactivity in vis and UV light: The influence of calcination temperature and surface properties

A series of TiO₂ photocatalysts were obtained using several calcination temperatures ranging from 350 to 750 °C. The photocatalysts’ characteristics by X-ray diffraction, UV–vis and FTIR diffuse reflectance spectroscopies, X-ray photoelectron spectroscopy, BET and BJH methods showed that sample active in vis region had anatase structure, about 200 m²/g specific surface area, absorbed light for λ > 400 nm and contained 10.1 at.% of C–C species. The photocatalytic activity of the catalysts was estimated by measuring the decomposition rate of phenol in 0.21 mM aqueous solution in visible and ultraviolet light. The experimental data clearly indicate correlation between the absorption intensity of irradiation by obtained powders and their photocatalytic performance in phenol degradation. An increase in absorbance over the entire vis region and the highest photocatalytic activity for phenol degradation in visible light (λ > 400 nm) occurred for photocatalyst calcinated at 350 °C. Photocatalyst processed at 450 °C had the best activity in UV light (250 < λ < 400 nm).     

Synthesis of doped ceria with mesoporous flowerlike morphology and its catalytic performance for CO oxidation

Mesporous flowerlike ceria Ce_0.9M_0.1O_2−δ (M = Y, La, Zr, Pr and Sn) have been synthesized successfully by a hydrothermal method. The impacts of doping on their physical properties are investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption, Raman and X-ray photoelectron spectroscopy (XPS). The doped materials show relatively high stability against the grain growth at 800 °C under reducing and oxidizing atmosphere. The catalytic activities of all flowerlike ceria materials for CO conversion are quite high (200 < T₅₀ < 320 °C) due to their high specific surface area (>100 m²/g), open mesoporous structure (pore size  3.9 nm) and nano-crystalline nature (grain size < 10 nm). Among flowerlike materials, Pr and Sn doped ceria show enhanced activity while La, Y, Zr doped ceria show decreased activity compared to undoped ceria.     

Preparation of copper-containing mesoporous manganese oxides and their catalytic performance for CO oxidation

Copper-containing mesoporous manganese oxides were prepared by the sol–gel method. The samples obtained were characterized by XRD, N₂ adsorption–desorption, ICP, CO-TPD, redox measurement and XPS. After calcination at 300 °C, amorphous structure was shown by XRD for all the samples. All the samples had mesopores of about 6 nm and high surface areas of 170–230 m² g⁻¹. Using these samples as catalysts, CO oxidation was carried out as a model reaction. Copper-containing mesoporous manganese oxide prepared by the sol–gel method showed a very high activity. On the other hand, copper-supported manganese oxide prepared by the impregnation method using copper sulfate showed a low activity. Differences in activities were correlated with the mobility of lattice oxygen.     

Polymer-derived SiOC aerogel with hierarchical porosity through HF etching

Silicon oxycarbide (SiOC) aerogels have been synthesized from preceramic polymers via pyrolysis in inert atmosphere at 1200 and 1300 °C. The as synthesized materials have a typical colloidal microstructure with mesoporosity in the range 10–50 nm and no microporosity. HF acid attack of the SiOC aerogels dissolves preferentially the SiO₂-rich phase and creates micro-and (small)mesopores (<10 nm) in the aerogels microstructure finally leading to a materials with hierarchical porosity. The HF post-pyrolysis treatment is more efficient for the SiOC aerogels pyrolyzed at the maximum temperature, i.e. 1300 °C, leading to a maximum value of specific surface area of 530 m²/g and total porosity of 0.649 cc/g.     

Methane activation by NO2 on Co loaded SBA-15 catalysts: The effect of mesopores (length, diameter) on the catalytic activity

In a general model of “three-function deNO_x” catalyst, the partial oxidation of methane by NO₂ is an important step (CH₄ + NO₂ → C_xH_yO_z + NO). To study the effect of the length and diameter, in the mesopores of SBA-15, we have synthesized catalysts with 3 wt.% cobalt supported on SBA-15, with differences in length and diameter of channels. Three different cobalt species were detected on all catalysts. We demonstrated by TPSR experiments that the activity of cobalt/SBA-15 catalysts is affected by the length, the diameter and connections between mesopores of the SBA-15 supports. We show that by changing textural properties of silica support the temperature of 100% conversion of NO₂ into NO can decrease by more than 100 °C.     

Characterization of sepiolite as a support of silver catalyst in soot combustion

Turkish sepiolite–zirconium oxide mixtures were applied as a support for the silver catalyst in a soot combustion. Sepiolite–Zr–K–Ag–O catalyst was characterized by XRD, N₂ adsorption, SEM, TPR-H₂ and EGA-MS. The combustion of soot was studied with a thermobalance (TG-DTA). The modification resulted in a partial degradation of the sepiolite structure, however, the morphology was preserved. The adsorption of N₂ of the modified sepiolite is a characteristic for mesoporous materials with a wide distribution of pores. The specific surface area S_BET equals 83 m²/g and the pores volume is 0.23 cm³/g. The basic character of the surface centers of sepiolite is indicated by CO₂ desorption (TPD-MS) at 170 °C and at about 620 °C due to a surface carbonates decomposition. The thermodesorption of oxygen at 650–850 °C indicates the decomposition of AgO_x phases at the surface. The presence of AgO_x phases is also confirmed by TPR-H₂ spectrum (low temperature reduction peak at 130 and 180 °C). The high-temperature reduction at about 570 °C is probably related to Ag–O–M phases on the support.The soot combustion takes place at T₅₀ = 575 °C. Without silver (sepiolite–Zr–K–O) T₅₀ = 560 °C but sepiolite modified with silver (sepiolite–Zr–K–Ag–O) undergoes the same process at T₅₀ = 490 °C.     

A simple and effective process for fabrication of Gd0.1Ce0.9O1.95 solid electrolyte ceramics

Ce_0.9Gd_0.1O_1.95 ceramics were prepared using a simple and effective process in this study. Without any prior calcination, the mixture of raw materials was pressed and sintered directly. The reaction of the raw materials occurred during the heating up period by passing the calcination stage in the conventional solid-state reaction method. More than 99.5% of theoretical density was obtained for Ce_0.9Gd_0.1O_1.95 sintering at 1500–1600 °C. Fine grains (<1 μm) formed in pellets sintered at 1450 °C. The homogeneity of grains increased with the sintering temperature. The grains grew to >4.5 μm in pellets sintered at 1600 °C. The reactive-sintering process is proved to be a simple and effective method in preparing Ce_0.9Gd_0.1O_1.95 ceramics for solid electrolyte application.     

Synthesis of hierarchical porous silicon oxycarbide ceramics from preceramic polymer and wood biomass composites

Hierarchical porous SiOC ceramics were successfully prepared using a polysiloxane preceramic polymer mixed with wood biomass by annealing at different temperatures under Ar atmosphere. These SiOC ceramics display a trimodal pore size distribution in the micro-, meso- (micropores + mesopores, 1.7–14 nm) and macro-size scale (1–15 μm). The mesopores and micropores mainly originate from the formation of large amounts of SiC crystals and nanowires, graphite-like microcrystallites, and nm-scale pores of ray parenchyma cells and pits of the wood biomass. The SiOC sample prepared at a higher temperature processes the specific surface area up to 180.5 m²/g. The specific surface area, pore volume and average pore width of the samples can be controlled by adjusting the pyrolysis temperature.     

Direct synthesis of graphitic carbon nanostructures from saccharides and their use as electrocatalytic supports

An easy method is described for fabricating graphitic carbon nanostructures (GCNs) from a variety of saccharides; i.e., a monosaccharide (glucose), a disaccharide (sucrose) and a polysaccharide (starch). The synthesis scheme consists of: (a) impregnation of saccharide with Ni or Fe nitrates, (b) heat treatment under inert atmosphere (N₂) up to 900 °C or 1000 °C and (c) oxidation in liquid phase to selectively recover the graphitic carbon. This procedure leads to GCNs with a variety of morphologies: nanopipes nanocoils and nanocapsules. Such GCNs have a high crystallinity, as shown by TEM/SAED, XRD and Raman analysis. The GCNs were used as supports for platinum nanoparticles, which were well dispersed (Mean Pt size  2–3 nm). Electrocatalysts thus prepared have electrocatalytic surface areas in the 70–95 m² g⁻¹ Pt range and exhibit high catalytic activities towards methanol electrooxidation.     

A new approach for modeling adsorption kinetics and transport of methane and carbon dioxide in shale

In this article, a new approach is proposed to investigate adsorption kinetics and transport of gases in shale. Due to co-existence of pores with different size in the shale, a set of adsorption processes happened in pores of different sizes are considered. A first-order multi-process model is developed, which can perfectly fit the adsorption kinetic data of CH₄ and CO₂ obtained at different temperatures. The modeling and pore characterization results indicate that an adsorption process happens in micropores/mesopores (<50 nm) and another adsorption process happens in macropores (>50 nm) in the Wufeng shale. Gas diffusion mechanism is dominant in micropores/mesopores, and gas seepage mechanism is dominant in macropores. The effective diffusivity of CO₂ is smaller than that of CH₄, because the adsorption of large amount of CO₂ in the pores hinders its diffusion. The coefficients related to the diffusion and seepage have no obvious trend with temperature.